LG, the
company that makes the Nexus 4 for Google,
told CNET UK it's due to high demand for the phone. Here's
the quote from LG's head of mobile in the UK Andy Coughlin:

[The Nexus 4] had proven extremely popular, and as
such retailers have been met with huge demand.

However, LG and Google won't say how many Nexus 4s there
were to begin with, so it's tough to tell if the phone keeps
selling out so quickly because the supply is low or because there
are an overwhelming number of people trying to buy one.